Composition for and method of chrome pickling of magnesium shapes



water rinse. very pronounced at 30 seconds or longer. .thus formedadheres to the chromate film and resists and often escapes removal bythe water rinse and remains as United States Patent l COMPOSITION FORAND METHOD OF CHROME PICKLING OF MAGNESIUM SHAPES Lawrence Whitby, BayCity, Mich, assignor to The Dow Chemical Company, Midland, Mich., acorporation of Delaware No Drawing. Application December 24, 1956 aSerial No. 630,049

1 Claims. (Cl. 148-62) This invention concerns an improved chrome picklesolution for forming protective chromate coatings on magnesium articles,and especially concerns the method of forming such coatings whicharesubstantially free from powdery deposits and which provides animproved etched surface for the formation thereon of additionalprotective coatings.

The term magnesium, as used herein, refers to magnesium; and alloys ofmagnesium of which magnesium comprises at least 85 percent by weight.The term chromate .coatings refers to coatings which contain a highpercentage of a chromium compound.

Magnesium articles, such as shapes formed by extruding, forging,casting, and molding operations and articles fabricated of such shapes,have been found to have increased resistance to the attack ofdeteriorating substances Such a process of forming in a bath of thesolution known as chrome pickle.

A problem has long persisted in the art of chrome picklingrdue to theattack upon the surface of the magnesium .a rticle. being treated by athin film of residual chrome pickle, known as drag-out, during theinterim of its transfer from the chrome pickle bath to the rinse waterwhen this interim exceeds about thirty seconds. This attack is at leastpartially due to the continued action of the drag-out causing the pHvalue to rise to 5 and above during the transfer time. A delay in thetransfer time .of about 5 seconds, after its removal from the picklingbath and prior to the Water rinse, is practiced in the art since suchdelay increases the uniformity ,of the coating and thereby makes animproved base for the application of coatings of paint, plastic and thelike. Delays of up to 15 or 20 seconds may be tolerated but the reactionduring the delay is so rapid that a delay of over 30 seconds hasheretofore been avoided Whenever possible. The reason for thecriticality of the period of transfer is due to the formation of aparticulate material on the chromate coating after a delay of about 30seconds which heretofore has not been adequately removable by the Theformation of the particulate material is The material a powder on thedry finished magnesiumarticle. The transfer must, therefore, be rapid ifan excessive residue of particulatematerial remaining after the waterrinse, is to be avoided.

In commercial practice, the limitation in the permissible transfer timebetween bath and rinse which is ,inherent in known practice isundesirable both from the l stan'dpoint of operation and of quality ofetch. As a re- "sult, therefore, the particulate deposit thus often2,887,418 Patented May 19, 1959 present in excessive amounts on thesurface of the treated article after the water rinse. Such deposit isobjectionable because it is a serious impediment to the adhesion to suchsurfaces of paint and plastic which are commonly applied as furthercoatings and is a fire hazard during grinding and milling of magnesiumarticles thus affected.

A number of unsuccessful, orb-ut partially successful, attempts havebeen made to lessen the tendency of such a particulate deposit, which isformed by the dragout chrome pickle during contact with air, to resistits removal therefrom by the water rinse. Among these attempts are theinclusion of buffering agents in the pickbe present in relatively largeamounts. The expense of such treatment has proved to be well nighprohibitive in practical operations. Another attempt to lessen. theamount of the particulate material adhering to the rinsed shapeis theaddition to the chrome bath of an inhibitor such as an acid fluoride ofsodium, potassium, or ammonium which inhibits the etching effect of thechrome pickle solution. This clearly is disadvantageous becausetheetching effect of the bath is necessaryfor good paint and plasticadhesion. As a matter of fact, when: an inhibitor is added in sufiicientamounts to sufiiciently dispel the residual particulate materialadhering to the rinsed shape and prevent its remaining thereon as atroublesome powder causing interference with subsepowder on the finishedmagnesium articles formed by the interaction of the magnesium and thedragout atan .in-

creasing pH value and yet obtaining the benefits of such action inbringing about animproved etch, by employing a suitably stablesurfactant as an additament in the chrome pickling bath.

By surfactant is meant herein a material composed of molecules having ahydrophobic group and hydrophilic group which, when added to a chromepickle solution, reduces the surface tension thereof. The chrome picklesolution containing a surfactant Within the scope of the invention has asurface tension of less than 70 dynes per centimeter in the chromepickle bath.

As used herein, a surfactant is considered stable if it does not loseits effectiveness by decomposition or otherwise within twenty-four hoursafter its addition to the chrome pickle. However, surfactants which. arestable for relatively long periods of time, say for at least 20 days,are preferred. For example, a surfactant which is not stable for as longas about twenty-four hours is not recommended because the bathcontaining it must be revivified by additions of the surfactant to thebath more frequently than is consonant with good operation. Al-

though such additions are relatively inexpensive, they introduce acontrol problem which makes the use of the the structure: R--Ar-SO Mwhere Ris: a C toC alkyl radical, AI is a naphthalene or benzene group,and Mis an alkali metal. Sodium alkylnaphthalene isulfonate illustratesthis type.

(2) Alkyl sulphates, such as RC H -0SO Na where R is a C toC alkylradical. An example of thisfis sodium-lauryl sulfate.

(3) Sulfonated esters, e.g.,

(4) Alkali salts of alkylaryl polyether sulfonates. (5) Alkanesulfonatessuch as those represented by where R is an alkyl chain of C to C but ispreferably a C alkyl radical.

(6) Alkylphenyl polyethylene glycol ethers.

(7) Alkali salts of fluoro-substituted alkanesulfonates represented by RSO M where R is a perfluorinated alkyl radical having from 5 to 18carbon atoms and M an alkali metal or ammonium. Examples of this type ofsurfactant are CF (CF SO K and CF3(CF2)7SO3K.

(8) Those in which the hydrophilic fraction of the molecule is providedby ethylene oxide and the hydrophobic fraction is provided by, acarboxylic acid, RCOOH; by an aliphatic alcohol, ROH; a glycol, HOR'OH;a polyoxyalkylene glycol, HOROROH; by an alkylphenol, ArROH; a phenol,ArOH; or by a mercaptan, RSH. R is an alkyl chain consisting of 8 to 18carbon atoms and R' is an alkyl chain of 2 to 4 carbon atoms. An exampleof a reaction product of ethylene oxide and an alkylphenol is one havingthe general formula; n being an integer oar-[110w omormnon (9.)Alkylphosphates as represented by the expression:

general OR R-OP O OR where R is an alkyl group of 8 to 16 carbon atomsand R is a solubilizing group.

(10) Substituted amides formed by the reaction of a carboxylic acid andethanolamine, e.g.

(12) The condensation products of a carboxylic acid and an amine, e.g.

where .R is a C to C alkyl radical and R is an alkyl "radical of 2 to 4carbon atoms.

(13) Monoethers having the generic formula R(OR'),,(OR"),,,H

where R is an alkylchain of C or over, R is a C to C alkylene group, R"is the C alkylene group, n is 6 or more, and m has a value such that(OR) constitutes at least 25 percent of the total weight of themolecule.

In practicing the invention, the magnesium shape or article to be coatedis cleaned, if necessary, by immersing it in an aqueous alkalinesolution or by scrubbing it with such a solution in the customary manneremployed in preparing magnesium shapes for chromate coating. Usually thetemperature is between 190 to 212 F. and the period of cleaning from 3to 10 minutes. The solution may be prepared by dissolving about 3 oz. byweight of sodium carbonate and about 2 oz. by weight of sodium hydroxidein enough water to make a gallon of solution. After dipping or scrubbingthe magnesium, it is rinsed well with cool water. It is then treated bysubjecting it to the novel aqueous chrome pickle solution of theinvention at 70 to F. for 0.5 to 2 minutes. This solution of theinvention is prepared by dissolving in water from 9 to 40 percent byweight, but preferably about 18 percent by weight, of an alkali metalchromate, e.g., sodium dichromate, and from 4.5 to 60 percent by Weight,but preferably about 26 percent by weight, of 70 percent aqueous nitricacid (sp. gr. of about 1.41), and between 0.01 to 0.5 percent by weightof a surfactant as described hereinabove. The treated magnesium isremoved from the chrome pickle, exposed to air for from 15 to seconds,and rinsed thoroughly with water. The recommended procedure is to rinsefirst with cool water at a temperature from 40 F. to 70 F. andthereafter rinse with hot water at a temperature from about to F.

An improved chromate coating is thus formed on the surface of themagnesium shape by the action of the improved chrome pickle of theinvention. The chrome pickle containing a surfactant as defined hereinattacks the magnesium surface, and thereby produces a more uniform etchas shown by microscopic examination than do chrome pickles wherein nosurfactant is used. The coatings thus formed are readily rinsedsubstantially free from discrete particles on the surface. Furthermore,the coated magnesium piece need not be unduly hurried in its transferfrom the pickle to the rinse. A time well beyond 30 seconds may now beconveniently employed to produce a superior etch according to myinvention. A period of 0.5 to 1.0 minute is usually employed, but aperiod up to 2 minutes may be employed, to give excellent results.

Examples of the practice of the invention with various surfactants aredescribed below, pertinent data as to the surfactants used and resultsobtained being set forth in the accompanying table.

For each of the examples, prior to treatment according to the invention,a 3" x 6" x 0.064" test piece of a magnesium alloy, consistingessentially by weight of 96 percent magnesium, 3 percent aluminum, and 1percent zinc, was prepared by dipping it for 5 minutes in an alkalinebath at 200 F., the bath being made according to the followingformulation:

Sodium carbonate (Na CO .l0H O), 3 oz. by weight; Sodium hydroxide(NaOH), 2 oz. by weight; Water sufficient to make 1 gallon of solution.

Sodium dichromate (NaCr O .2H O), 710 ,g.;

70 percent nitric acid, sp. gr. 1.4, 1065 g.;

A surfactant as tabulated and sufficient water to make about 4 liters ofsolution at room temperature.

The pieces were removed from the solution, allowed to drain verticallyfor 60 seconds and then rinsed for .30

seconds in cool Water at about 55 F., thereafter immersed in hot waterat 170 F. for 5 seconds and dried at room temperature for one hour. As aresult, on the surface of each test piece, a chromate coating was formedwhich was substantially uniform in thickness and coloring andsubstantially free from powdery residue due to adhering particulatematerial. To determine the amount of powdery residue adhering thereon,if any, each test piece was thereafter weighed and vigorously rubbedwith :clean dry' cotton and then reweighed to determine the loss ofweight due to the removal of any powdery material which had formed onthe surface. The losses so .de- Rtermined in repeat runs were averaged.

As a blank or control, two repeat runs were made in which like pieces ofmagnesium alloy were used except no surfactant was present in thepickle. The weight of powder removed inthe two repeat runs was averaged.The average weight of powder removed from the control samples ispresented in the table as the blank.

Various surfactants were selected from different types and testedaccording to the above described procedure. The type and amount ofsurfactant used and the average weight of powder removed from 1 to 4samples of 3" x 6" x 0.064" pieces of the magnesium alloy treatedaccording to the invention are set out as Examples 1 to 30 in the table.

TABLE Efiects on chromate coating of difierent surfactants in varyingamounts Gone. in No. 01' Avg. Wt.

Example Surfactant in Pickling Bath Wt. Per- Samples of Powder cent Runin Grams None 2 0519 Potassium perfiuoro n 0. 01 2 0186 oetanesulfonate.100% N a lauryl sulfate 0. 025 4 0058 (Stepanol ME).

do 0.05 4 0087 do 0.1 4 0057 -do 0. 5 4 0076 100% solubllized alkylplans 0. 1 1 0262 phate (Victawet 12). 25% Na-ethylhexene sulfon- 0.0254 0181 ate (Tergitol EH).

0.1 4 0050 n 0.2 2 i 0139 n 0. 5 2 .0135 40% Na 2eth lhexyl sulfate0.025 3 .0151

(Tergitol 08 do 0.05 3 0165 d 0. 1 3 0137 100% polyoxyalkylene glycol 0.01 4 0168 ether (Tergitol XD) do 0. 025 4 0065 ---do 0. 4 0114 d0 0.1 40025 100% allk lvl plienyll t liloly- 0.05 4 0085 0 et ene co e er. jtio.z %Z H-i 0.1 i 100% alkyl polyoxyet y ene 0. 01

glycol ether (Tergitol TD).

21 do 0.025 4 0204 do 0.05 4 .0266 .....(lo 1 0.1 gl

100% alkylaryl o yether 0. 025

alcohol (Triton 102). do 0.05 4 0218 do 1 0.1 4 100% alkylaryl p0 yether0. O5 4 alcohol (Triton X-100). d0 0.1 4 .0081 28% N a alkylarylpolyether 0. 05 4 0214 sulfonate (Triton X-200). do 0. 1 4 0146 In theabove table, the examples of the invention show that the presence of thesurfactant employed in the chrome pickling bath definitely reduces theamount of powdery material retained on the surface of magnesium articleswhich have been treated in the chrome pickle bath, trans ferred throughair to a Water rinse, and rinsed in the water, even though the transfertime is as much as ninety seconds. This period of time is over 3 timesas long as the maximum time usually permissible for such transfer when aconventional chrome pickling bath is used.

losing its effectiveness after the bath containing it had been usedforover days.

EXAMPLE 31 Magnesium alloy pieces which had been treated in the chromepickle containing 0.1 weight percent of the 25 percent strength Na2-ethylhexene sulfonate according to Example 8 of the table and thosewhich were treated in the same chrome pickle except it did not contain asurfactant, i.e., prepared according with thefprocedure followed in theblank of the table, where subjected topaint durability tests. In thesetests, a primer coating which was prepared according to United StatesGovernment specifications No. MIL6889A, Type 1, was applied to themagnesium alloy pieces and top-coated with an aluminumpigmentednitrocellulose lacquer. The zinc chromate primer is known to possessonly fair adhesion to magnesium alloys after aging. All pieces wereexposed to like natural outdoor weathering conditions. After 12 monthsof such exposure, the adhesion of the paint over the straight chromatecoating was only fair whereas that over the chromate coating formed bythe chrome-pickling solution as modified by addition of the surfactantaccording to the invention was excellent. Exposure was continued for atotal of 18 months. A scratch test was then used to evaluate the degreeof adhesion or strength of the bond formed between the paint and themagnesium metal. The degree of adhesion was ascertained according to across-hatch scratch test. This test consisted of making, by means of asteel needle, a series of scratches through the paint at approximately/2 millimeter intervals and repeating such scratches at right angles toand over those previously made, thus preparing a number of squares ofpainted surface of /2 millimeter sides bounded by scratch lines. Thescratches were made while the panel was under observation with alow-power binocular microscope. Complete flaking of the paint over theconventional chromate coating took place when it was sub jected to thescratch test, whereas such test caused no flaking of the paint over thechromate coating formed by treatment with the chrome solution modifiedby the surfactant according to the invention.

The paint durability as shown by the scratch test demonstrates that thereduction in powdering and improved etching of magnesium alloys treatedwith a chrome pickle containing a surfactant in accordance with theinvention results in a marked improvement in the firm retention ofpaints applied thereover, particularly when the primer paint is onewhich manifests increasingly poor adhesion to magnesium surfaces withaging upon exposure to natural outdoor conditions.

From the foregoing examples of the invention, it is not to be inferredthat the invention is to be limited to the chrome pickling solutionscontaining surfactants named therein, as the invention contemplatesbroadly the employment of a surfactant which is soluble and stable in achrome pickling solution, which permits a longer period of exposure toair during transfer of a magnesium article from the solution of a waterrinse and substantially decreases the particulate material adhering tothe article.

Having described the invention, what is claimed and desired to beprotected by Letters Patent is:

1. In a process of chromate coating of a magnesium article by subjectingthe article to the action of a chrome pickle solution consistingessentially of an aqueous solution of nitric acid and an alkali metaldichromate the step of admixing in said chrome pickle solution between0.01 and 0.5 percent by weight of a surfactant selected from the classconsisting of alkylaryl sulfonates, alkyl sulfates, alkanesulfonates,halo-substituted alkane sulfonates, sulfonated esters, and alkylarylpolyether sulfonates.

-2. The processes according to claim 1 wherein said alkali metal saltsof a halo-substituted alkanesulfonates is an alkali metal salt of aperfluoroalkanesulfonate having from 5 to 18 carbon atoms.

3. The process according to claim 2 wherein said alkali salt of ahalo-substituted alkanesulfonate is the potassium salt ofperfluoro-n-octanesulfonic acid.

4. An aqueous chrome pickling solution for treating articles ofmagnesium and its alloys, said solution containing 3 to 40 percent byweight of nitric acid and from 9 to 40 percent by weight of a dichromateselected from the group consisting of ammonium and alkali metaldichromates, and from 0.1 to 0.5 percent by weight of a surfactantselected from the class consisting of the alkali metal salts ofalkylaryl sulfonates, alkyl sulfates, al-

kanesulfonates, halo-substituted alkanesulfonates, sulfonated esters,and alkylaryl polyether sulfonates.

5. The pickling solution according to claim 4 wherein thehalo-subsituted alkanesulfonate is an alkali metalpe'rfluoroalkanesulfonate having from 5 to 18 carbon atoms in the alkanegroup.

8 6. The pickling solution according to claim 5 wherein the alkali metalis potassium.

7. The pickling solution according toclaim 6 wherein the potassiumperfluoroalkanesulfonate is potassium perfiuoro-n-octanesulfonate havingthe formula:

CF (CF SO K References Cited in the file of this patent UNITED STATESPATENTS 2,301,983 Tanner Nov. 17, 1942 2,321,948 Shawcross June 15, 19432,548,420 Chester Apr. 10, 1951 2,610,133 Thomson Sept. 9, 1952 FOREIGNPATENTS 287,450 Great Britain Oct. 18, 1928 921,686 France Jan. 13, 1947OTHER REFERENCES Metals and Alloys, February 1945, pages 417 -434. Page423 relied on.

1. IN A PROCESS OF CHROMATE COATING OF A MAGNESIUM ARTICLE BY SUBJECTINGTHE ARTICLE TO THE ACTION OF A CHROME PICKLE SOLUTION CONSISTINGESSENTIALLY OF AN AQUEOUS SOLUTION OF NITRIC ACID AND AN ALKALI METALDICHROMATE THE STEP OF ADMIXING IN SAID CHROME PICKLE SOLUTION BETWEEN0.01 AND 0.5 PERCENT BY WEIGHT OF A SURFACTANT SELECTED FROM THE CLASSCONSISTING OF ALKYLARYL SULFONATES, ALKYL SULFATES, ALKANESULFONATES,HALO-SUBSTITUTED ALKANE SULFONATES, SULFONATED ESTERS, AND ALKYLARYLPOLYETHER SULFONATES.